What exactly does this mean? It means that when you, for example, earn 8 copper coins from the PTCWall an additional 8 experience points will be added to your character. This goes for all offers including video's.

This means you could level up your character by watching video's when you're not at your computer, since you only get kicked out of the game when you're idle for more than 30 minutes and not from the offer wall page.